1. Technical Field of the Invention
The present invention relates generally to an air-fuel ratio control system for internal combustion engines, and more particularly to such a system designed to perform feedback control on an air-fuel ratio of the engine using outputs of air-fuel ratio sensors.
2. Background Art
There are known air-fuel ratio control systems for internal combustion engines which have air-fuel ratio sensors installed upstream and downstream of an exhaust emission control catalytic device disposed in an exhaust pipe of the engine and use outputs of the air-fuel ratio sensors to control the air-fuel ratio of the engine for enhancing the emission control. For example, Japanese Patent First Publication No. 2-67443 discloses such a system. There are also known anther type of air-fuel ratio control systems which have a first catalytic device and a second catalytic device arrayed in upstream and downstream sides of an exhaust pipe and a first and a second air-fuel ratio sensors installed upstream and downstream of the first catalytic device, and use outputs of the first and second air-fuel ratio sensors to control the air-fuel ratio of the engine for enhancing the emission control. For example, Japanese Patent First Publication No. 5-321651 discloses such a system.
The former systems must be increased in size of the catalytic device in order to ensure a desired degree of emission control using the single catalytic device. The use of the two air-fuel ratio sensors disposed upstream and downstream of the catalytic device may also result in a lack of a control response rate. The latter systems have a difficulty in monitoring the exhaust gas emitted ultimately outside the exhaust pipe (i.e., from the second catalytic device), thus resulting in a deterioration of the emissions.
There are further known air-fuel ratio control systems which have a first catalytic device and a second catalytic device arrayed in upstream and downstream sides of an exhaust pipe, a first air-fuel ratio sensor installed upstream of the first catalytic device, a second air-fuel ratio sensor interposed between the first and second catalytic devices, and a third air-fuel ratio sensor installed downstream of the second catalytic device. The systems also include a first, second, and third feedback controllers. The first feedback controller works to bring the air-fuel ratio of a mixture supplied to the engine into agreement with a target one using an output of the first air-fuel ratio sensor in feedback control. The second feedback controller works to determine a parameter controlled by the first feedback controller using an output of the second air-fuel ratio. The third feedback controller works determine a parameter controlled by the second feedback controller using an output of the third air-fuel ratio. For example, Japanese Patent First Publication No. 8-14088 (U.S. Pat. No. 5,537,817) discloses such a system.
The above type of control systems are designed to perform the feedback control three times, thus resulting in complexity of the system structure. Additionally, time lags of response to flows of the exhaust gas downstream of the catalytic devices occur, thereby causing the second and third feedback controller to interfere in operation with each other, which leads to the instability of the operations thereof. For example, when the air-fuel ratio varies at high frequencies, it may cause the output of the second air-fuel ratio sensor to have a fuel rich value and the output of the third air-fuel ratio sensor to have a fuel lean value. This may cause the second and third feedback controller to interfere in operation, thus resulting in the instability of the operations thereof.